A conventionally proposed linear motor comprises a magnet member composed of round-shaft-like permanent magnets having alternately arranged respective N poles and S poles, and a coil member having a plurality of coils arranged in an axial direction so as to surround the periphery of the magnet member (for example, the Unexamined Japanese Patent Application Publication (Tokkai-Hei) No. 10-313566).
The linear motor disclosed in the Unexamined Japanese Patent Application Publication (Tokkai-Hei) No. 10-313566 is used, for example, in OA equipment or the like in place of a combination of a rotary motor and a ball screw. However, using the linear motor in a machine tool such as a press machine which makes direct-acting motion also results in a simple configuration and enhanced controllability.
However, while the linear motor is performing a direct-acting operation, where a magnetic substance is present around the outer periphery of a magnet member in a route of the linear motor, an attractive force varies at a period when the magnet member passes through each magnetic pole of the magnet member. This varies a resulting thrust. The magnetic substance may be provided, for example, in order to prevent the magnet member from falling down. Where the linear motor is used as a driving source for a machine tool such as a press machine, the variation in attractive force varies the speed of the contact between a workpiece and a tool. The variation in contact speed is reflected in the processing quality such as a press cut surface or a machining processing surface. Thus, the processing quality is degraded.
An object of the present invention is to provide a linear motor device that can reduce a variation in thrust caused by a variation in attractive force. Another object of the present invention is to provide a machine tool such as a press machine which can reduce the degradation of the processing quality caused by the variation in attractive force, while using the linear motor as a driving source for simplification.